The invention relates to a switched-mode power supply for converting a DC input voltage into an output voltage having a trigger circuit for triggering at least one controlled switch that periodically applies the input voltage to at least one primary winding of a transformer, it being possible to switch off an auxiliary voltage for the trigger circuit using an auxiliary semiconductor switch when the input voltage drops below a given minimum value, the auxiliary voltage being, on the one hand, a first auxiliary supply voltage derived from the input voltage and, on the other hand, a second auxiliary supply voltage derived from an auxiliary winding of the transformer via rectification.
In switched-mode power supplies of the standard type, the trigger circuit draws the auxiliary supply voltage required for its operation first during startup from an auxiliary voltage capacitor as the first auxiliary supply source situated above a high-resistance current path from the input voltage, in part during the ramp-up but after that from an auxiliary winding of the transformer after rectification and regulation via an in-phase regulator as a second auxiliary power supply. A constant voltage supply of the trigger circuit via the input voltage, which is usually attained via rectification of an AC supply voltage and often is called a link circuit voltage, would cause undesirable losses in the series resistance because the input voltage may be as much as 370 volts or more, but the supply voltage of the trigger circuit is only 15 volts, for example, so the voltage difference must be across the series resistor, and even at only low current consumption of the trigger circuit of, e.g., 10 mA, the power to be dissipated would be a few watts.
The starting point of the problem on which the present invention is based is that in lowering the DC supply voltage below a given value, e.g. 100 volts, the voltage supply of the trigger circuit must be interrupted in order for the switched-mode power supply to be shut down. Otherwise, proper operation would not be possible; the limits of the pulse duty factor of the trigger pulse for the switch, excessive current etc. would be encountered. Furthermore, the aforementioned threshold voltage value at which the circuit is to be switched off or switched back on must be subject to hysteresis; otherwise, instabilities would result.
The switching of a switched-mode power supply according to the prior art, in which the aforementioned switching off or switching back on of the supply voltage of the trigger circuit is possible, is shown in FIG. 1. A DC supply voltage Uzk, which was acquired, for example, via rectification of an AC mains voltage and whose nominal range is between, for example, 120 and 370 volts, can be switched via a controlled switch S to a primary winding Wp of a transformer UET. The connected current in this current flows via a sensor resistor Rsh from which information corresponding to the current is supplied to a trigger circuit AST, which supplies a control pulse to the gate electrode of the controlled switch S. Secondarily, the voltage of a secondary winding Ws of the transformer is rectified using a rectifier diode Dg, filtered at a capacitor Ca and is available as output voltage Ua. The information of a voltage sensor SPS at output voltage Ua is fed via an optocoupler OKO of the trigger circuit AST, so that a regulation of the output voltage via a corresponding change of the pulse duty factor of the trigger pulse for switch S is possible. In addition, it is also possible to regulate to a voltage in the primary circuit, as well as to an output current or an input current, these kinds of regulation and their combinations being well known to an expert in the field.
The trigger circuit AST has an input H for its operating voltage, hereinafter referred to as auxiliary supply voltage. This auxiliary supply voltage is attained, as already mentioned, in two different ways. During the startup phase of the trigger circuit or the switched-mode power supply, very generally a voltage is available that is present across an auxiliary voltage capacitor Cs and is attained using a high-resistance current path, in this case a high-resistance series resistor Rs. This voltage is designated here as Uh and it is routed across the breaker gap of an auxiliary semiconductor switch Ts to aforementioned voltage supply input H of trigger circuit AST. During the operation of the power supply, an auxiliary coil Wh supplies a voltage that is rectified using a rectifier diode D and filtered at a capacitor C. This voltage is likewise routed through an in-phase regulator LAE and a decoupling diode De to supply input H of trigger circuit AST.
In order to enable the hysteresis-affected switching off and on of auxiliary switch Ts, which in this case has a base-emitter resistance Rbe, a comparator KOM is provided whose output is routed to the switch input of auxiliary switch Ts, in this case to the base. The positive input of comparator KOM is present at the reference voltage Uref produced by a Zener diode Zref. Zener diode Zref receives its current via a resistor R at which auxiliary supply voltages Uh or Uhl are present. Therefore a voltage is present on the positive input of comparator KOM that corresponds to the current auxiliary voltage. A voltage is present at the negative input of comparator KOM that is obtained from input voltage Uzk using a voltage divider Ra, Rb. In the positive feedback branch of comparator KOM, a resistor network R41, R42 is provided that determines the hysteresis behavior of comparator KOM.
If, during this switching of the switched-mode power supply, input voltage Uzk drops below a value prescribed by reference diode Zref, e.g. 100 volts, comparator KOM switches off auxiliary switch Ts and along with it the auxiliary supply voltage of trigger circuit AST, which thereupon no longer cycles and the entire switched-mode power supply stops its power supply operation. If supply voltage Uzk thereafter increases again, comparator KOM connects auxiliary switch Ts again, but with an hysteresis of, for example, 20 volts, which then occurs corresponding to the aforementioned exemplary hysteresis at an input voltage of 120 volts. Even though the desired function of the switching-off or switching-back-on part is ensured by this circuit, the relatively high current consumption (power loss) of the needed components is disadvantageous, as is the space required for the components.
One object of the invention is to find a circuit for the controlled switching on and off of the auxiliary supply voltage that has a lower current consumption and is simpler in construction.